This section is intended to provide a background or context to the material disclosed below. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived, implemented or described. Therefore, unless otherwise explicitly indicated herein, what is described in this section is not prior art to the description in this application and is not admitted to be prior art by inclusion in this section. Abbreviations that may be found in the specification and/or the drawing figures are defined below, at the end of the text of this specification.
In an LTE RAN, the Physical Layer Cell ID (PCI) is used by a User Equipment (UE) to distinguish between different cells of the same center frequency as the UE makes cell measurements. Therefore, neighboring cells of the same center frequency should not have the same PCI values so that the UE can distinguish between all of the neighbor cells. That is, a same PCI collision, where two cells have the same PCI, is avoided. The PCIs of the nearby cells reported by the UE to the cell are used by the cell for some RRM procedures such as handovers. This requires neighbor-of-neighbor cells of the same center frequency to not have the same PCI values (i.e., the same-PCI collision or confusion must be avoided). Since the number of PCIs is limited to 0 (zero) to 503 values and same-PCI collisions and confusions must be avoided, PCI assignment is considered to be one of the most tedious phases in LTE RAN planning and optimization.
For a time-synchronized, single-frequency-layer LTE network with two or more Tx antenna ports for each cell, cells with the same PCI modulo 3 value have the same time-frequency locations of the Cell-specific Reference Signals (CRS). Since the CRS have the same time-frequency location, the CRSs of the adjacent cell interfere to the UE of the serving cell. Laboratory and field tests have shown that CRS-to-CRS interference results in 10 percent or greater drop in downlink throughput at the cell-edge compared to CRS-to-PDSCH interference, even with Block Error Rate (BLER)-based Channel Quality Indicator (CQI) correction. The CRS-to-CRS interference also potentially leads to poorer call success rates, handover success rates, and other KPIs.
The same PCI modulo 3 value also produces the same Physical Synchronization Signal (PSS) Sequence. Interference among the PSS leads to poorer detection of the presence of neighbor cells. To avoid interference between the CRSs and misdetection of the PSS, it is desirable for neighboring cells of the same center frequency to have different PCI modulo 3.
When there is a single Tx antenna port for each cell or there are two-antenna ports are in a Vertical-Horizontal (V-H) polarization configuration, the CRSs of cells with the same PCI modulo 6 values have the same time-frequency-polarization locations. To avoid interference of these CRSs which lead to degradation in performance, for this scenario, it is desirable for neighboring cells of the same center frequency to have different PCI modulo 6.
For cells with the same PCI modulo 30, these cells have the same PUCCH Demodulation Reference Signal (DMRS) sequence group number. Cells with the same DMRS group number cause more uplink interference with each other. Hence, to avoid excessive uplink interference, it is desirable for neighboring cells of the same center frequency to have different PCI modulo 30.
Because of the potential losses of having un-optimized PCI allocations in the network, it would be beneficial to improve the PCI allocations.